Estimation of Evapotranspiration in the Mesoregion of the Lower São Francisco

Abstract One of the most important factors in any water resource management program is having the knowledge about the natural processes that regulate the hydrological cycle. The aim of this study was to estimate the reference Evapotranspiration (ETo) in the Baixo São Francisco watershed (BSF), using the Penman-Monteith (PM) method recommended by the Food and Agriculture Organization (FAO), in the period 2009 to 2018. The first step of the research was to fill in gaps in surface data, using the statistical predictive mean method (Predictive Mean Matching-PMM). After validation of the method, it was possible to estimate ETo using the PM-FAO method, where it was found that the temporal dynamics of ETo in most study locations follow a similar pattern, having only different values in their maximums and minimums. Regarding the average daily ETo values, Pão de Açúcar, in Alagoas, had the highest value in the study region, 4.69 mm.day−1 and Arapiraca, also in Alagoas, the lowest value with 4.18 mm.day−1. The average of the accumulated annual total of ETo of the BSF for the period was 1635.23 mm.year−1. The low ETo observed during the rainy season (April to July) is associated with lower temperatures. The work obtained good results, if compared with other studies already conducted in lower San Francisco.摘要 水资源管理项目中最为重要的影响因素之一，是掌握调控水文循环的自然过程相关知识。本研究旨在采用联合国粮食及农业组织（Food and Agriculture Organization, FAO）推荐的彭曼-蒙特斯（Penman-Monteith, PM）方法，对圣弗朗西斯科河下游流域（Baixo São Francisco watershed, BSF）2009年至2018年的参考蒸散量（reference Evapotranspiration, ETo）进行估算。本研究的首要步骤为采用统计预测方法——预测均值匹配（Predictive Mean Matching, PMM）填补地表数据的缺失值。在完成该方法的有效性验证后，即可通过PM-FAO方法开展ETo估算，结果显示，多数研究站点的ETo时间动态变化模式趋于一致，仅极值存在差异。就日均ETo值而言，位于阿拉戈斯州的帕苏迪阿苏卡尔站点为研究区域内最高值，达4.69毫米/天；同样地处阿拉戈斯州的阿拉皮拉卡站点则为最低值，为4.18毫米/天。研究时段内圣弗朗西斯科河下游流域的年均累积ETo均值为1635.23毫米/年。研究区域雨季（4月至7月）观测到的较低ETo值与气温偏低显著相关。将本研究结果与此前针对圣弗朗西斯科河下游地区开展的相关研究对比可知，本研究取得了良好的估算效果。